In the field of road traffic which tends to increase daily, research has been carried out to facilitate as far as possible the flow of the vehicles by the opening of detours when traffic slows down significantly on account of the increasing number of vehicles which accumulate on a roadway or on a section thereof. The facilities which may thus be offered to road traffic to render it more fluid are based on the knowledge of a certain number of data concerning the vehicles, which data may include the presence of a vehicle, its passage at a given place, possibly its speed, its direction, and the like.
All these items of information may be conventionally obtained, from magnetic detectors or sensors wherein a transmitter radiates electromagnetic energy, which can be picked up by one or more receiver coils forming part of the detector. A metallic mass (e.g., a vehicle) entering the field of radiated energy disturbs the latter and gives rise to a signal in the output of the receiver coils.
A known magnetic detector of this type comprises a transmitter coil and two receiver coils coaxial therewith which are weakly coupled to the transmitter coil and are connected in opposition to each other whereby, in the absence of a conductive mass in the field produced by the transmitter coil, the voltages induced in the receiver coils are in balance. The presence of a metallic mass in the field produces a modification therein which is manifested by the appearance of a finite voltage in the output of the receiver coils. This output voltage is utilized in load circuits connected on the downstream side of the detector.
A detector of the type just described, however, has a number of drawbacks due prinicipally to the collinearity of the receiver coils to their weak mutual coupling which require relatively large physical dimensions of the detector. Even with a precise mechanical presetting of the three coils with respect to one another it is possible, however, that an output signal is generated in the absence of an intruding object by the natural environment of the road.